C-arm units are widely used in medical applications. With regard to these systems, the X-ray detector and the X-ray source are mounted opposite one another on a so-called C-arm which is designed to perform a rotational movement. This means that the imaging system with the X-ray source and the X-ray detector can rotate around a rotation center, the so-called isocenter of the C-arm system. In this manner, not only two-dimensional X-ray images can be obtained when using modem C-arm systems but, by rotating the imaging system around the patient, three-dimensional, CT-like images or cross-sections can also be obtained. The image reconstruction of the three-dimensional images or of the cross-sections from the X-ray images recorded using different rotation angles or projections takes place in a similar manner to a computer tomograph (CT). One rotation of the C-arm around the patient through an angle range of 180° plus the fan angle of the X-ray beam is adequate for this purpose. Soft body parts or, through subtraction of contrast agent images and native images, angiograms can thus be represented three-dimensionally. A C-arm unit of such a type is described in US 2006/0120507 A1 for example.
In addition to these common C-arm systems, robot-guided C-arm systems are also known for example from DE 10 2005 012 700 A1, in which a C-arm with an X-ray source and an X-ray detector situated opposite the X-ray source is coupled to the hand of a robot which moves this X-ray recording system around the object of interest in order to record the two-dimensional X-ray images.
The X-rayed target area, of which a three-dimensional image is subsequently obtained by means of reconstruction, is restricted as a result of the size of the beam cone of the X-ray beam. The size of the beam cone is in turn coordinated with the size and the distance of the X-ray detector, as a rule a flat detector. The target area of interest must lie in the isocenter of the C-arm system in order that it can be imaged by the imaging system for each rotation or projection angle. With regard to target areas of interest in medical imaging, which are located centrally in the patient cross-section, this condition is met without further ado. However, if other areas are to be represented, then problems result on account of the above condition. It is thus not possible in many practical situations, for example with regard to vertebroplasty or kyphoplasty, to carry out the rotation of the C-arm on a circular path around the desired imaging area since collisions would occur in this case between the C-arm and the patient, the patient support or instruments in use.